Postfix is a mail transfer agent (MTA). Its job is to handle the delivery of the mail: pick it up and send it to the next point on its route. Postfix is a postal worker, whose job is to take an envelope and (with help from its colleagues) carry it to the recipient.

What you're asking for here is secretarial work: assembling documents to put them in the envelope. That's not Postfix's job: it's a job for a mail user agent (MUA). You can write a crude MUA that just assembles pieces to make a mail in a few lines of shell, as grawity did, but Mutt is really a good tool for this task.